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59 lines
2.9 KiB
59 lines
2.9 KiB
15 years ago
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<sect1 id="ai-julianday">
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<sect1info>
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<author>
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<firstname>John</firstname>
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<surname>Cirillo</surname>
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</author>
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</sect1info>
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<title>Julian Day</title>
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<indexterm><primary>Julian Day</primary>
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</indexterm>
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<para>
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Julian Days are a way of reckoning the current date by a simple count of
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the number of days that have passed since some remote, arbitrary date. This
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number of days is called the <firstterm>Julian Day</firstterm>,
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abbreviated as <abbrev>JD</abbrev>. The starting point, <abbrev>JD=0</abbrev>,
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is January 1, 4713 BC (or -4712 January 1, since there was no year '0'). Julian
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Days are very useful because they make it easy to determine the number of days
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between two events by simply subtracting their Julian Day numbers.
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Such a calculation is difficult for the standard (Gregorian) calendar, because
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days are grouped into months, which contain a variable number of days, and
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there is the added complication of <link linkend="ai-leapyear">Leap
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Years</link>.
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</para><para>
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Converting from the standard (Gregorian) calendar to Julian Days and vice versa
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is best left to a special program written to do this, such as the &kstars;
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<link linkend="tool-calculator">Astrocalculator</link>. However, for those
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interested, here is a simple example of a Gregorian to Julian day converter:
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</para><para>
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<abbrev>JD</abbrev> = <abbrev>D</abbrev> - 32075 + 1461*( <abbrev>Y</abbrev> +
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4800 + ( <abbrev>M</abbrev> - 14 ) / 12 ) / 4 + 367*( <abbrev>M</abbrev> - 2 -
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( <abbrev>M</abbrev> - 14 ) / 12 * 12 ) / 12 - 3*( ( <abbrev>Y</abbrev> + 4900 +
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( <abbrev>M</abbrev> - 14 ) / 12 ) / 100 ) / 4
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</para><para>
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where <abbrev>D</abbrev> is the day (1-31), <abbrev>M</abbrev> is the Month
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(1-12), and <abbrev>Y</abbrev> is the year (1801-2099). Note that this formula
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only works for dates between 1801 and 2099. More remote dates require a more
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complicated transformation.
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</para><para>
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An example Julian Day is: <abbrev>JD</abbrev> 2440588, which corresponds to
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1 Jan, 1970.
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</para><para>
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Julian Days can also be used to tell time; the time of day is expressed as a
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fraction of a full day, with 12:00 noon (not midnight) as the zero point. So,
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3:00 pm on 1 Jan 1970 is <abbrev>JD</abbrev> 2440588.125 (since 3:00 pm is 3
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hours since noon, and 3/24 = 0.125 day). Note that the Julian Day is always
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determined from <link linkend="ai-utime">Universal Time</link>, not Local Time.
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</para><para>
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Astronomers use certain Julian Day values as important reference points, called
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<firstterm>Epochs</firstterm>. One widely-used epoch is called J2000; it is the
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Julian Day for 1 Jan, 2000 at 12:00 noon = <abbrev>JD</abbrev> 2451545.0.
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</para><para>
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Much more information on Julian Days is available on the internet. A good
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starting point is the <ulink
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url="http://aa.usno.navy.mil/faq/docs/JD_Formula.html">U.S. Naval
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Observatory</ulink>. If that site is not available when you read this, try
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searching for <quote>Julian Day</quote> with your favorite search engine.
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</para>
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</sect1>
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